Voltage compensating tap changing transformers are used by utility power companies in power substations to respond to varying load demand by electric power consumers. Such variable tap or tap changing transformers are voltage compensating transformers whose secondary tap may be changed in response to varying demand in power consumption in order to maintain substantially constant output voltage.
Tap changing transformers generally have a rotatable shaft for changing the secondary tap of the transformer to different secondary tap positions or tap numbers by rotation of the shaft. A shaft motor rotates the shaft through different tap changing shaft angles in response to control signals.
According to the state of the art the position angle transducer comprises a shaft angle sensor for generating shaft angle analog signals corresponding to the tap changing shaft angles. Typically the shaft angle sensor is a synchronous generator having rotor and stator coils operatively coupled for generating rotor and stator synchro signals corresponding to the tap changing shaft angle. Alternatively, the shaft angle sensor may be a resolver having rotor and stator coils coupled for generating resolver signals corresponding to the tap changing shaft angle.
The shaft angle analog signals are used by the utility power companies to provide information on the secondary tap positions or tap numbers of the respective substation transformers. This information is useful in utility load planning and may provide input data to the Supervisory Control and Data Acquisition (SCADA) System of the utility company. The SCADA System in turn can provide control signals to the respective shaft angle prime movers or motors of the transformers.
A difficulty is encountered with present position angle transducers if the relationship between the tap changing shaft angle and the tap number or tap position is non-linear for a particular tap changing transformer. That is, a change in the shaft angle may not result in a proportionate change in tap number or tap position. In that event, the utility company cannot rely upon the shaft angle analog signals to reflect the operative secondary tap position or tap number of the transformer.
Furthermore, the transfer function or functional relationship of shaft angle to tap number may vary with different transformers from different manufacturers or even between transformers from the same manufacturer. The conventional position angle transducers suffer from this transfer function variability between the actual tap numbers and the shaft angle analog signals input to the SCADA System.